1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically, an image forming apparatus wherein a photosensitive member charged with a specified potential beforehand is exposed to a laser beam responding to image data so that an electrostatic latent image is formed on the photosensitive member, and then the latent image becomes visible on a sheet of paper.
2. Description of Related Art
Generally, regarding an image forming apparatus wherein an electrostatic latent image on a photosensitive member is caused by exposure to light with image data, and then the latent image appears on a paper sheet, an electrophotographic copier wherein a copy of an original results from reflection and a laser printer wherein a laser beam is turned on and off in accordance with image data are known. These apparatuses are different from each other only in measure of exposure, and the other elements of an image forming section except a light source can be commonly used in the apparatuses.
Therefore, Japanese Laid Open Patent Publication No. 54-25736 suggests an image forming apparatus for which exposure means utilizing the reflection of an original and exposure means utilizing a modulated laser beam are both provided, wherein the other image forming elements are commonly used. This arrangement enables an image to be formed selectively by the reflected light from an original or the modulated laser beam.
In this apparatus, however, it is impossible to form a composite image by using both the reflected light from an original and the laser beam simultaneously. For, in this apparatus, the light to be reflected by the surface of an original and the laser beam are emitted from different light sources, and their optical paths are different except the last portion of them.
Further, in an image forming apparatus with a function of printing with a modulated laser beam, when a scanning device which is equipped with a plurality of equally divided deflecting surfaces such as a polygon mirror is used for scanning the laser beam in the main scanning direction in order to compensate an error in formation of each deflecting surface, a sensor for detecting a starting point of printing in each scan is so arranged that the focus of the laser beam on the sensor is equivalent to that on the photosensitive member. However, when forming a composite image, it is impossible to use the sensor as it is.